A comparator of the kind disclosed in, e.g., the specification of Japanese Patent Kokai (Laid-Open) Publication JP-A-5-14073 has been proposed as a comparing and amplifying detector circuit in the prior art. The comparator, which uses a differential amplifier, employs depletion-type MOS transistors as amplifying elements and is adapted in such a manner that a digital signal is obtained even for an input voltage in the low-level range. FIG. 10 is a diagram showing a circuitry of the comparator disclosed in this publication. A constant current, which is obtained by a depression-type transistor 201 whose drain is connected to a power supply and whose gate and source are tied together, is input as a reference current to the input side of enhancement-type transistors 202, 207 that construct a current mirror circuit. An output current of the current mirror circuit is supplied as a driving current to transistors 204, 206 forming a differential pair and having sources that are tied together, gates to which input terminals 10, 11 are respectively connected and drains that are respectively connected to load elements (depletion-type transistors whose drains are connected to the power supply and each of whose gate and source are tied together) 203, 205. A drain of the transistor 206 is connected to the input of an inverter comprising a depletion-type transistor 208 and an enhancement-type transistor 209, namely to a gate of the transistor 209. A signal applied to this inverter is inverted and amplified thereby and output at an output terminal 12.
A circuit of the kind shown in FIG. 12 is known as a comparator used in electrical equipment in automobiles. The comparator has a power supply 1, a control IC 13 arranged on control ground 16; a battery 222, a low-side PID (intelligent power device) 220 and a load 221 arranged on power ground 18: interface resistors 211, 212, 214, 215, 217 and 218; transistors 213, 216; and a voltage regulator diode 219.
In the operation of the circuit shown in FIG. 12, the output voltage of the control IC 13 is potentially divided by the resistors 211, 212 and the resulting voltage is applied to the base of the NPN transistor 213. By turning the transistor 213 on and off, the PNP transistor 216 is turned on and off, the voltage of the battery 222 is clamped by the voltage regulator diode 219 and a signal is applied to an input of the low-side IPD 220.
The specification of Japanese Patent Kokai (Laid-Open) Publication JP-A-6-244414 discloses an arrangement serving as a temperature detector circuit in which, as shown in FIG. 13(A), a constant current obtained by a depletion-type transistor 223 flows into a diode group of serially connected diodes 224, and a potential at a connection node between the depletion-type transistor 223 and the diode group 224 is input to a gate of an enhancement-type transistor 226 and is compared with a threshold voltage VT of the transistor 226, whereby temperature is detected utilizing the temperature dependence illustrated in FIG. 13B.